Induction of Autophagy to Achieve a Human Immunodeficiency Virus Type 1 Cure

Abstract

Effective antiretroviral therapy has led to significant human immunodeficiency virus type 1 (HIV-1) suppression and improvement in immune function. However, the persistence of integrated proviral DNA in latently infected reservoir cells, which drive viral rebound post-interruption of antiretroviral therapy, remains the major roadblock to a cure. Therefore, the targeted elimination or permanent silencing of this latently infected reservoir is a major focus of HIV-1 research. The most studied approach in the development of a cure is the activation of HIV-1 expression to expose latently infected cells for immune clearance while inducing HIV-1 cytotoxicity-the "kick and kill" approach. However, the complex and highly heterogeneous nature of the latent reservoir, combined with the failure of clinical trials to reduce the reservoir size casts doubt on the feasibility of this approach. This concern that total elimination of HIV-1 from the body may not be possible has led to increased emphasis on a "functional cure" where the virus remains but is unable to reactivate which presents the challenge of permanently silencing transcription of HIV-1 for prolonged drug-free remission-a "block and lock" approach. In this review, we discuss the interaction of HIV-1 and autophagy, and the exploitation of autophagy to kill selectively HIV-1 latently infected cells as part of a cure strategy. The cure strategy proposed has the advantage of significantly decreasing the size of the HIV-1 reservoir that can contribute to a functional cure and when optimised has the potential to eradicate completely HIV-1.

Keywords: HIV-1; SMAC mimetics; apoptosis; autophagy; autosis; cell death; latency promoting approach; nanoparticle.

Figure 1

The initiation of autophagy is a multistep process. The main regulators of autophagy are MTOR, an inhibitor, and AMP-activated kinase (AMPK) an activator. MTORC1 inhibition induces autophagy through the formation of the ULK1 complex. This complex initiates the formation of the phagophore by directly activating the phosphatidylinositol 3-kinase catalytic subunit type 3 complex I (PIK3C3 complex). The PIK3C3 complex translocates to endoplasmic reticulum sites and produces phosphatidylinositol 3-phosphate (PI3P) which then recruits PI3P-binding proteins such as WIPI2 and ZFYVE1. HIV-1 Nef can inhibit this process by enhancing the association between BECN1 and its inhibitor BCL2. In the ATG12 conjugation system, ATG12 forms a covalent bond with ATG5, which then binds ATG16L1, followed by dimerization (not shown) and interaction with the PI3P-binding complex. The WIPI2b isoform acts immediately upstream of ATG16L1 and recruits ATG12–ATG5-ATG16L1 to PI3P-containing phagophores. The ATG12–ATG5-ATG16L1 complex then promotes conjugation of ATG8 family proteins with phosphatidylethanolamine (PE), which are then incorporated into phagophore membranes, where they interact with cargo receptors harbouring LC3-interacting motifs. ATG16L1, as well as the polyubiquitin-binding autophagy receptors including SQSTM1, OPTN, NBR1, and NDP52 also recruit and incorporate ubiquitin-decorated cargos into autophagosomes including HIV-1 Tat, HIV-1 Vif, and HIV-1 p24. After detachment of ATG factors, the isolation membrane closes through membrane scission, forming the autophagosome. These, then, fuse with lysosomes resulting in the degradation of the engulfed components as well as the ATG8-PE and SQSTM1 associated with the inner membrane. HIV-1 Nef affects autophagosome maturation by preventing the fusion between autophagosomes and lysosomes.

Figure 2

Potential strategy to achieve an HIV-1 cure. (a) Antiretroviral therapy (ART) suppresses HIV-1 plasma viremia, but upon interruption of ART, it quickly rebounds; (b) conventional HIV-1 Tat/P-TEFb inhibitors fail to silence epigenetically the HIV-1 promoter, thus when they are removed, transcription is eventually restored and HIV-1 viremia rebounds; (c) supplementation of ART with a latency-promoting epigenetic silencing drug (LPA) could promote a state of deep latency, but as they do not reduce the viral reservoir, rebound could occur; (d) supplementation of ART with both an LPA and a drug that selectively kills HIV-1 infected cells could decrease the size of the viral reservoir, significantly prolonging the time to viral rebound after ART interruption. Optimization of such a strategy has the potential to completely eliminate the viral reservoirs.

Figure 3

DIABO/SMAC mimetic mediated apoptosis. Treatment of latent HIV-infected cells with DIABLO/SMAC mimetics induces the degradation of XIAP and BIRC2/3, inducing autophagy, the deubiquitination of RIPK1, and the disinhibition of caspases. This leads to the formation of a death inducing signalling complex comprising of FADD, RIPK1, RIPK3 and caspase-8 on autophagosomal membranes, resulting in the cleavage and activation of caspase-8, thus enabling activation of the effector caspases (caspase-3 and caspase-7) and the execution of apoptosis.